1. Field of the Invention
The present invention relates generally to fluid-filled vibration damping devices suitably used as an automobile engine mount, body mount, differential mount, and the like, and more particularly to a fluid-filled vibration damping device capable of exhibiting vibration damping effect based on vibration damping characteristics exhibited by flow action of the fluid sealed therein.
2. Description of the Related Art
As one type of vibration damping couplings or mounts, there is known a fluid-filled vibration damping device that is capable of exhibiting vibration damping effect on the basis of resonance or flow action of the non-compressible fluid sealed therein.
In order to meet a demand to damp vibration of plural types, such a fluid-filled vibration damping device has been arranged to have an orifice passage for providing damping action effective for vibration of low frequency band, and a movable member for providing damping action effective for vibration of high frequency band in combination. JP-B-4-17291, JP-A-1-164831, JP-A-3-121327 and JP-A-2003-148548 show specific examples of such vibration damping devices, for example. These devices may be used as an automobile engine mount, and will exhibit excellent vibration damping effect for both low-frequency and large-amplitude vibration such as engine shakes, and high-frequency and small-amplitude vibration such as booming noises.
Meanwhile, these fluid-filled vibration damping devices having such a conventional structure, may suffer from relatively large vibration transmission or impulsive noises created when an impulsive substantial vibrational load is applied between the first mounting member and the second mounting member. For instance, when the conventional device is used as an automobile engine mount, such vibration and noises may be created upon engine cranking or abrupt acceleration of the vehicle.
The present inventors have conceived that the relatively large vibration and impulsive noises may be created when gas once separated from the sealed fluid due to an excess negative pressure generated in a pressure receiving chamber where an amount of flow of the fluid through the orifice passage is limited, are again dissolved in the sealed fluid.
To address the aforementioned problem, the present assignee has been proposed in JP-A-2003-148548 a modified fluid-filled vibration damping device wherein additionally provided is a short passage to make the orifice passage short, and a valve means for closing the short passage. In this device, when a negative pressure in the pressure-receiving chamber excessively increases, the valve means is operated to open the short passage, thereby eliminating excess increase of the negative pressure in the pressure-receiving chamber.
However, the proposed structure disclosed in JP-A-2003-148548 needs not only to incorporate the valve means but to control operation of the valve means precisely, thereby needing a sophisticated construction and maintenance of the device. Therefore, there is a room for improvement for reducing the proposed device to practice.